1. Technical Field
The present invention relates to a suppression method for corrosion of a carbon steel member and more particularly, to a suppression method for corrosion of a carbon steel member, suitable for a nuclear power plant.
2. Background Art
A boiling water reactor plant (hereinafter referred to as a BWR plant) and a pressurized-water reactor plant (hereinafter referred to as a PWR plant), for example, are known as power plants. The BWR plant, for example, has a nuclear reactor disposing a core in a reactor pressure vessel (hereinafter referred to as a RPV). Cooling water supplied to the core by a recirculation pump (or an internal pump) is heated using the heat generated by nuclear fission of nuclear fuel material in fuel assemblies loaded in the core, and part of the cooling water turns into steam. This steam is introduced to a turbine from the reactor to drive the turbine. A generator coupled to the turbine rotates to generate power. The steam exhausted from the turbine is condensed by a condenser and turns into water. This water is supplied to the reactor as feed water. In order to suppress radioactive corrosion products from being generated in the reactor, a demineralizer is provided to a feed water pipe to remove mainly metallic impurities from the feed water.
In a power plant such as the BWR and PWR plants, stainless steel and nickel base alloy are used for a wetted surface of main structure members such as a reactor pressure vessel, to suppress corrosion. However, in some structure member such as a reactor coolant clean-up system, a residual heat removal system, a reactor core isolation cooling system, a core spray system, and a feed water system, the structure members made of carbon steel are mainly used to reduce construction cost of the plant or to avoid stress corrosion cracking of stainless steel caused by high-temperature water flowing in the feed water system, etc.
Unfortunately, the carbon steel members composing the reactor coolant clean-up system, the residual heat removal system, the reactor core isolation cooling system, the core spray system, and the feed water system, also have a wetted surface come into contact with water, and this wetted surface may corrode. In this case, if the carbon steel member is disposed downstream of a clean-up apparatus, corrosion products from the carbon steel member may cause radioactive corrosion products in the reactor. Furthermore, the corrosion products from the carbon steel member may decrease the heat-transfer efficiency of a secondary system in a PWR plant.
In order to suppress corrosion of a carbon steel member composing a plant, some methods have been proposed such as, for example, a method for forming an oxide film on a surface of the carbon steel member by injecting oxygen in a feed water system of the plant, and a method of adding chemicals such as ammonia (NH3) and hydrazine (N2H4) in a feed water system of the plant to obtain alkaline pH (see Japanese Patent Laid-open No. 2000-292589, for example).
In the conventional method of injecting oxygen in feed water of the plant, however, the oxygen needs to be continuously injected during the operation of the plant since the effect of corrosion control cannot be obtained when the oxygen injection is stopped. Moreover, in view of suppressing corrosion in the plant, the method is against a recent preference of keeping the reactor in a reducing environment.
In the conventional method of adding chemicals in feed water of the plant to obtain alkaline pH, described in Japanese Patent Laid-open No. 2000-292589, the chemicals need to be continuously added during the operation of the plant in the same way. Moreover, the added chemicals increase burdens on a condensate clean-up apparatus, a clean-up apparatus for a reactor clean-up system, and so on, which may cause the amount of waste from the clean-up apparatuses to be increased.
To solve these problems, a method for forming a closely-packed ferrite film (for example, a magnetite [Fe3O4] film or a nickel ferrite [NiFe2O4] film) on a surface of a carbon steel member in a BWR plant, for example, has been proposed (see Japanese Patent Laid-open No. 2007-182604, for example). In this proposal, a film-forming aqueous solution including a first agent containing iron (II) ions, a second agent containing nickel ions, a third agent (oxidizing agent) for oxidizing the iron (II) ions into iron (III) ions, and a fourth agent (a pH adjustment agent) for adjusting pH is used for forming a nickel ferrite film. The ferrite film serves as a protection film for blocking cooling water from contacting the carbon steel member, so that the surface of the carbon steel member come into contact with the cooling water is suppressed from corrosion.
In addition, a method for forming a ferrite film on the inner surface of a recirculation pipe made of stainless steel in a BWR plant is disclosed in Japanese Patent Laid-open No. 2006-38483.